Chemical vapor deposition processes such as pyrolytic processes and hydrolytic processes are well known in the art of coating substrates. The physical characteristics of the coating reactants utilized in such processes may be a liquid, a vapor, or a solid dispersed in gaseous mixtures, aerosols, or vaporized or vaporous coating reactants dispersed in gaseous mixtures.
In the process of deposition of a vaporized chemical compound on a glass substrate in the production of photovoltaic devices, the vaporized chemical compound is typically deposited in a vacuum atmosphere as described in U.S. Pat. No. 5,248,349 to Foote, et al.; U.S. Pat. No. 5,945,163 to Powell, et al.; and U.S. Pat. No. 6,676,994 to Birkmire, et al. The systems for carrying out such process have typically included a housing having an enclosed deposition chamber that includes a vacuum source for drawing a vacuum within the deposition chamber. The vacuum deposition chamber typically includes heaters for heating the glass sheets as they are passed through the system. The glass sheets pass into the deposition chamber from a vacuum-heating furnace to the vacuum deposition chamber that is maintained at a similar vacuum and temperature setting as the heating furnace.
Powdered cadmium sulfide and powdered cadmium telluride are fed into the vaporization deposition chamber. The films are then deposited onto the previously coated and heated glass substrates sequentially. The cadmium telluride thin-film material requires a follow-on processing step to re-crystallize its polycrystalline structure so that effective photovoltaic devices can be made from the film stack.
Another method of depositing a vaporized chemical compound on a glass substrate for the production of photovoltaic devices is disclosed in commonly owned U.S. patent application Ser. No. 11/573,768 for ATMOSPHERIC PRESSURE CHEMICAL VAPOR DEPOSITION to Johnston in which the deposition occurs at atmospheric pressure within a heated inert gas filled furnace as the glass is passed through the furnace. Individually metered masses of semiconductor material, preferably cadmium sulfide (CdS) or cadmium telluride (CdTe) in powdered form, are introduced into a zone which is continuously purged by a stream of inert gas, preferably nitrogen, flowing between an inlet and an outlet at approximately atmospheric pressure. The powder is carried from the inlet, by the inert gas flowing at a controlled rate, into a heated vaporizer consisting of a refractory material. In the vaporizer, the powder is vaporized to form a mixture of the hot inert gas and the vaporized powder material. The outlet of the heated vaporizer is caused to communicate with the interior of a heated zone to impinge the vaporized material upon a surface of the substrate.
In order to control the thin-film deposition rate of the vaporized material and fluid emitted from the apparatus that is applied to the substrate, the mass flow rate of the fluid mixture and the velocity of the substrate are controlled while controlling the temperature of the substrate below the condensation point of the vaporized material. As the heated fluid/material mixture impinges onto the cooler substrate, it cools to a temperature below the condensation temperature of the vaporized material. The material condenses from the fluid mixture, in a polycrystalline form, onto the moving substrate as a continuous thin-film layer.
It has been found that thin-film coating systems, based upon the above referred to technologies, are capable of depositing thin-film of cadmium sulfide/cadmium telluride photovoltaic material onto commercially available soda-lime glass substrates in a vacuum and at atmospheric pressure. The photovoltaic materials are subsequently treated to re-crystallize the cadmium telluride surface making the film stack ready for further processing into photovoltaic devices.
However, the described deposition processes under a vacuum and at atmosphere each involve a single pass of the material vapor from a vapor generating system over the substrate to obtain the thin-film thereon. The deposition rate of the material on the substrate is dependent upon the rate of vapor molecules impinging the substrate surface. For single pass deposition, the concentration of vapor molecules must be sufficiently high to achieve the required deposition thickness from at least one stream of vapor from the vapor generating system. However, when the vapor concentration of the material becomes higher than the supersaturation level of the vapor at the operating temperature and pressure of the process, loose dust may form due to vapor phase nucleation of the semiconductor material.
Accordingly, it would be desirable to develop a thin-film photovoltaic material deposition process adapted to minimize the vapor phase nucleation of the semiconductor material during the deposition process to maximize the quality of the thin-film formed on a substrate, while minimizing the cost of production thereof.